The present invention relates to a motor driving circuit having a PWM driving circuit for changing an on-off ratio of a pulse signal so as to control driving currents flowing in plural phases of driving coils. Particularly, the present invention relates to a motor driving circuit which can prevent the influence of high frequency noise generated in a PWM driving circuit.
There is known a motor driving circuit having a PWM driving circuit for changing an on-off ratio of a pulse signal so as to control driving currents applied to plural phases of driving coils. In such a motor driving circuit, the frequency of the pulse signal in the PWM driving circuit is usually set to be a high frequency of dozens kHz or more. In addition, the changes of currents in the PWM driving circuit, that is, the leading and trailing edges of currents are made sharp. Thus, high frequency noise is generated from the PWM driving circuit.
When this high frequency noise gets into a circuit section other than the motor driving circuit, the driving currents flowing into the driving coils fluctuate, thereby causing a malfunction of the motor. In the related art, any of the following measures is taken to avoid such a harmful influence.
First, the PWM driving circuit is shielded to prevent high frequency noise from radiation. Second, the changes of the currents are made gentle enough to restrain high frequency components from being generated, so that high frequency noise generated in the circuit is itself lowered. Third, a snubber circuit (for example, filter condensers) for absorbing noise is added into a circuit in which a current having a sudden change flows, so that high frequency components are absorbed by this snubber circuit.
However, any of the measures has its own problem. First, in the method in which the PWM driving circuit is shielded, it costs much to provide a shield. In addition, it is impossible to give a perfect shield to the PWM driving circuit.
Second, in the method to make the changes of the currents gentle, the loss in driving transistors connected to the driving coils increases so that the efficiency deteriorates. In addition, since the loss increases, large-capacity ones must be adopted as those transistors. Such transistors are so expensive as to increase the cost of the driving circuit.
Third, in the case where a snubber circuit is provided for absorbing noise, it is necessary to add new electric/electronic elements to the driving circuit, so that the cost of the driving circuit is likewise increased. In addition, it is necessary to give careful consideration onto the characteristics of elements to be used in the snubber circuit, in order to be able to absorb high frequency noise satisfactorily.